Electrospray processes have become an important means of producing highly charged droplets and gas phase ions.(1) A particularly useful application of the electrospray process is the production of gas phase ions from analytes in liquid solutions delivered by high pressure liquid chromatography, capillary electrochromatography or capillary electrophoresis to a mass spectrometry for detection and analysis. Electrospray processes have been observed for solutions with positive and negative needle potentials which result in positive and negative net charge on droplets, respectively. The charged droplets from the electrospray process evaporate and ultimately eject ions into the gas phase from the solution.(2)
The electrospray process, in its simplest geometric form, is represented by the "classic cone-jet" represented in FIGS. 1 and 2. We define a classic cone-jet as one in which no appreciable discharge or gas breakdown occurs in the gases surrounding the electrospray cone-jet. FIG. 1 shows the electrospray process presented as an integral part of an electrical circuit. In the bulk solution, current flows via migration of anions and cations. On the surface of the cone-jet and droplets, current flows via motion of the liquid carrying a net charge. In the other regions of the circuit, electrons move through conductors. Electron transfer reactions occur at the interfaces between the solution and the conductors to maintain charge balance in the circuit [denoted in FIG. 1 as needle electrode and collection electrode]. In the ionization region some of the charge is carried by motion of gas-phase ions produced by electrospray ionization. When operating electrospray with a positive needle potential, an oxidation reaction occurs at the needle electrode and a reduction reaction occurs at the collection electrode. When operating electrospray with a negative needle potential, a reduction reaction occurs at the needle electrode and oxidation occurs at the collection electrode. It should be noted that in the case of conductive needles, the needle itself serves as the needle electrode. In FIGS. 1 and 2 the solution electrode is shown to be discrete from the needle in order to better illustrate the location of the redox reactions and the motion of the ions in solution relative to the electrode. These Figures would be representative of operation with insulated needles.
FIG. 2 shows a schematic diagram of an expanded view of an electrospray cone-jet operating in the positive mode. The arrows indicate the general direction of electrophoretic migration of anions [A-] and cations [C+] in a solution. In this particular case the anions are migrating in the direction of the needle electrode [anodes]. The shaded area of the cone-jet represents the gas-liquid interface where a net positive charge resides due to the depletion of anions or enrichment of cations at the surface of the liquid. The liquid is accelerated in the cone region by a high electrical field at the surface until liquid motion overcomes surface tension. At this point a liquid jet emerges from the apex of the cone with a net positive charge from excess cations on the surface.
FIG. 3 shows a current-voltage plot for electrospray of methanol indicating the boundaries of stable cone-jet operation. Most solvents sprayed with electrospray exhibit a qualitatively similar behavior to methanol where the stable cone-jet is bracketed by a lower voltage limit represented by an onset voltage [V.sub.onset ] and an upper voltage limit [V.sub.breakdown ] represented by a discharge voltage. The specific values for V.sub.onset are a function of the solution, flow, and system geometry. While the specific values for V.sub.breakdown are a function of pressure, surrounding gas composition, and system geometry.
The amount of current collected at the collection electrode in this classic mode of electrospray is highly dependent upon the nature of the solution being sprayed. The deformation of the liquid resulting in the cone-jet geometry is a balance between the forces holding the liquid onto the end of the needle [intermolecular forces of the solution] and the forces driving the ions on the surface of the liquid toward the collection electrode [motion of ions in field gradients]. The net charge in electrospray is due to the migration of ions [cations and anions] through the solution relative to the rate of removal of liquid from the tip of the needle. Both processes are governed by the properties of the liquid.
Some recent experiments by Tang and Gomez (3,4) show that a stable cone-jet is also observed in the presence of a corona discharge. This new regime of stable operation of electrospray cone-jets occurs at lower voltages and higher observed current than classic cone-jet mode and was denoted the corona-assisted mode of electrospray. These experiments suggest that an external source of ions, as they observed from their experimental system under conditions of spontaneous corona discharge, may lead to sensitivity enhancements in electrospray. They attributed their results to increased charging of droplets by (CO.sub.2)H.sup.+, (CO.sub.2).sub.2.sup.+, and (CO.sub.2).sub.2 H.sup.+, observed with mass spectrometry. They suggest that the higher current they observed was a consequence of the collection of these gas-phase ions onto the liquid and droplet surfaces.
The present invention is also a method and an apparatus for generating external gas-phase ions for interacting with electrospray processes in order to enhance charging of the droplets.